Compressors, such as rotary compressors, are well known devices used typically for compressing refrigerant in a refrigerating apparatus having a vapor compression refrigerating cycle refrigerant circuit. A conventional rotary compressor comprises a closed-type casing in which there are mounted a compression mechanism and an electric motor operable to activate the compression mechanism.
The compression mechanism includes a cylinder and a piston which rotates or revolves in an eccentric motion while substantially making contact with an inner peripheral surface of the cylinder. Defined between the cylinder and the piston is a compression chamber. A blade, mounted in the compression chamber, divides the compression chamber into two parts, i.e., a low pressure side (suction side) part and a high pressure side (compression/discharge side) part. One of the low and high pressure side parts of the compression chamber is switched to serve as the other in alternating manner every time the piston makes a single revolution.
When the compressor electric motor is powered, a low pressure gas refrigerant is sucked towards the low pressure side part of the compression chamber from an evaporator of the refrigerant circuit. At the same time, in the high pressure side part of the compression chamber a gas refrigerant is compressed to a high pressure level, and is discharged into the casing. The gas refrigerant present in the inside of the casing flows outwardly from a discharge pipe of the compressor, and is delivered to a condenser of the refrigerant circuit. While being switched in alternation, the high and low pressure side parts of the compression chamber repeatedly carry out the aforesaid operations, and the compression mechanism substantially continuously sucks in a low pressure gas refrigerant and, at the same time, discharges a high pressure gas refrigerant.
The compression mechanism is provided with a discharge opening through which high pressure gas refrigerant is discharged into the casing from the compression chamber. A discharge valve mechanism is mounted at the discharge opening. Generally the discharge valve mechanism includes a reed valve as a valve element. When the pressure in the high pressure side part of the compression chamber exceeds the pressure in the casing by more than a predetermined value, the resulting pressure difference between the high pressure side part and the casing causes the reed valve to carry out an operation of placing the discharge opening in the open state. On the other hand, when gas refrigerant flows outwardly from the compression chamber and enters the casing, the high pressure side part is switched to serve as the low pressure side part. As a result, the aforesaid pressure difference diminishes, thereby causing the reed valve to carry out an operation of placing the discharge opening in the closed state.
However, there is an overcompression loss problem with the above-mentioned discharge valve mechanism. More specifically, when the lift amount of a reed valve is small or when the opening area of a discharge opening is small, the resistance against the passage of a high pressure gas refrigerant increases. This results in compressing a refrigerant to excess in the compression chamber. On the contrary, if the lift amount of the reed valve is increased, this causes lags in closing the valve element. As a result, a high pressure gas refrigerant flows backwards from the casing into the compression chamber. Accordingly, there is the possibility that a volumetric efficiency drop occurs. On the other hand, if the opening area of the discharge opening is increased, the reed valve, too, increases in size and its mass increases. This may result in a valve opening response drop causing an overcompression loss.
If the opening area of the discharge opening is increased to a further extent, the refrigerant that has once been compressed expands again. This produces a new problem that the efficiency of the compressor falls off (re-expansion loss). More specifically, during the refrigerant discharge operation, there exists a certain amount of high pressure refrigerant remaining in the volume of the discharge opening. If the opening area of the discharge opening is increased, this results in increasing the amount of high pressure refrigerant remaining within the discharge opening. Accordingly, the amount of refrigerant which expands in the compression chamber after discharge becomes greater and, as a result, there occurs a drop in compression efficiency.
Consequently, in order to deal with such problems, a discharge valve mechanism which makes use of a poppet valve shaped like a circular cone has been proposed (see for example Japanese Patent Application Kokai Publication No. 2001-289254). A part of a valve element is fitted into a discharge opening. In the discharge valve mechanism disclosed in this patent publication, the poppet valve is employed for the purpose of gaining a discharge opening area. In addition, the poppet valve is used with a view to inhibiting the occurrence of overcompression or re-compression losses by reducing the amount of gas refrigerant remaining in the discharge opening after discharge.
Problems to be Solved
However, even the use of a poppet valve may produce some problems. That is to say, if the operating displacement is made variable by making an electric motor variable in speed by inverter control, the rate of flow of gas refrigerant at the discharge opening increases or decreases. This presents overcompression loss problems.
More specifically, a valve element originally designed for use at small flow rate is, as a rule, small in lift amount as well as in opening area. If such a valve element is used at a large flow rate, the velocity of flow increases because of lift amount insufficiency and opening area insufficiency. This results in an increase in discharge resistance, thereby producing overcompression losses. On the other hand, if a valve element originally designed for use at large flow rate is used at a small flow rate, the valve element is of a size larger than is required for such a small flow rate. In addition, the flow velocity becomes slower and, as a result, the level of pressure acting on the valve element falls off. Consequently, the response during the opening operation deteriorates. After all, an overcompression loss occurs.
Bearing in mind the above-mentioned problems with the prior art techniques, the present invention was made. Accordingly, a general object of the present invention is to inhibit the occurrence of overcompression losses in operations from small to large displacement in a variable displacement compressor and to achieve improvements in compressor operating efficiency.